Iron-chromium-nickel-carbon-nitrogen heat-enduring alloy steels



Dec. 3, 1940. Q HARDER ETAL 2.2Z3,659

IRON-CHROMIUM-NICKEL-CARBON-NITROGEN HEAT-ENDURING ALLOY STEELS FilBd Jan. 15, 1940 5 Sheets-Sheet 2 INVENTOR Oscar E.Har er:

JamesT Gow- ATTORNEYS 3, 1940- o. E. HARDER ETAL 2,223,659

IHON-CHROMIUM-NICKEL-CARBON-NITROGEN HEAT-ENDURING ALLOY STEELS Filed Jafi. 15, 1940 5 Sheets-Sheet 3 INVENTORS Oscar E. Harden James T Gow- ATTORNEYS Dec. 3, 1940. o. E. HARDER EIAL IRONCHROMIUMNICKEL CARBON-NITROGEN HEAT-ENDURING ALLOY STEELS Filed Jan. 15, 1940 5 Sheets-Sheet 4 INVENTORS Oscar E. Harder? James T Gow B HM ATTORNEYS i Dec. 3, 1940. o. E. HARDER EIAL IRON-CHROMIUM-NICKEL-CARBON-NITROGEN HEAT-ENDURING ALLOY STEELS Filed Jan. 15, 1940 5 Sheets-Sheet 5 IrZwO mun I ZmmvOEIEZ BY James T Gow.

ATTORNEYS Patented Dec. 3, 1940 UNITED STATES IRON-CHROBHUM-NICKEIFCARBON-NITRO- GEN HEAT-ENDURING ALLOY STEELS Oscar E. Harder and James T. Gow,

Ohio, assignors to Alloy Columbus, Casting Research Institute, Inc., New York, N. I, a corporation of New York Application January 15, 194i), Serial 14Claims.

The present invention relates to iron-chro mium-nickel-carbon-nitrogen heat-enduring alloy steels of improved ductility. It pertains, particularly, to the improvement of ductility of such steels at and after exposure to elevated temperatures.

In the present state of the art, the heat-resisting or heat-enduring alloys contain, in combination with the major elements,ir0n, chromium and nickela wide range of carbon, silicon and manganese contents, and at times for various reasons, such additional elements as molybdenum, tungsten, cobalt, columbium, titanium and aluminum. The minor elements, carbon, silicon and manganese, are virtually indispensable in sand cast alloys; the carbon and silicon increase the castability of the alloys, the silicon and manganese act to some extent as deoxldizers, and the carbon also exerts a beneficial influence on certain of the mechanical properties. The relatively high carbon'content of the heat-resisting or heat-enduring alloys mainly differentiates them from the iron-chromium-nickel corrosion-resistant alloys used in the chemical industries which usually have as low a carbon content as it is practicable to produce, because carbides present in the alloys may induce rapid corrosion attack in their immediate vicinity, generally referred to as intergranular corrosion.

Governing the choice of a particular composition for a specific high temperature application are those factors relating to (a) the resistance to corrosive action of the gas atmosphere or other substances at elevated temperatures (b) the re- 35 sistance to flow or slow deformation under load at elevated temperatures (usually termed creep strength) (c) the ductility and stability of ductility under stress at elevated temperatures and (d) the room temperature ductility, or retained o ductility after exposure to elevated temperatures. Certain typical uses of the high-chromium,

low-nickel alloys are oil still tube supports in the oil refineries, and various industrial heat-resisting furnace parts and accesories, such as beams, mullles, baille plates, annealing boxes, conveyors, trays, et cetera.

The application of this type of alloy has been somewhat limited, due to a tendency of the alloy to embrittle when exposed within the moderate 50 temperature range of about 1100 F. to 1650 F. for but relatively short times, and at higher tern,- peratures on more extended periods of exposure. The embrittlement induced by exposure to moderate temperatures is evidenced most largely by tests at room temperature, while the embrittlement induced by exposure to the more elevated temperatures is often evidenced by fracturing occurring with but little deformation while the alloy is under stress at elevated temperatures.

"Also, the embrittlement induced by exposure to elevated temperatures may cause unexpected fracturing of the castings when they are subjected to mechanical abuse, thermal shock, or stresses set up by thermal gradients.

In recent years, the industry has earnestly endeavored to produce heat-resisting or heat-enduring alloys having relatively good creep strength and a high retained ductility after exposure at all temperatures within the range of 1200 F. to 2000 F. by using a chromium content which is high in relation to the nickel content of the alloy. For example, the industry has, in the main, turned toward alloys containing about per cent chromium and about 12 per cent nickel, in contrast with alloys containing relatively high percentages of nickel and relatively low percentages of chromium, as for example, 35 per cent nickel-45 per cent chromium alloys or approximately 60 per cent nickel-42 to 15 per cent chromium. So far as we are aware, however, no wholly successful solution of the production of heat-resisting or heat-enduring alloys for the conditions indicated has been developed.

The constitution of essentially carbon-free, iron-chromium-nickel alloys has received study by a number of investigators. A summary and correlation of previous investigations and of recent research conducted at the National Physical Laboratory is given in an article on The constitution of the alloys of nickel, chromium and iron by Jenkins and others, published in the Journal of the Iron and Steel Institute, vol. 136, 1937, page 187. A brief presentation of similar information is given by Bain in the American Society for Metals Handbook, 1939, pages 418-422.

In brief, it has been shown from the abovementioned researches that the ferritic phase may carry only a limited amount of nickel but approach 100% chromium, while the austenitic phase may carry but a limited amount of chromium but approach 100% nickel. The composition range of the austenite and ferrite phase is found to be altered to some extent by temperature. Except in the case of allows of very low chromium and nickel contents, the rate of transformation at temperatures below about 2000 F. is so sluggish that, with even relatively slow cooling, the structural phases set up at the high temperatures are preserved at room temperature. It is an object of the present invention to provide structural parts. for high temperature engineering applications made of an iron-chromiumnickel alloy which possess not only high creep strength but also possess superior ductility both at elevated temperatures and at room tempera,- tures after being subjected to heat.

Another object of our invention is to provide an austenitic high-chromium, low-nickel content alloy possessing resistance to corrosive fuel gas atmospheres and also possessing superior ductility both at elevated temperatures and at room temperatures after being subjected to heat.

Our present invention is concerned mainly with relatively high-chromium, low nickel content alloys of nominal compositions within the range of 20% to 30% chromium, 8% .to 15% nickel, balance substantially iron, which are characterized in general by relatively high strength properties, oxidation and sulphidation resistance at elevated temperatures, which properties make them especially applicable for structural parts for fuelfired industrial furnaces. This type of alloy is also of industrial interest since it possesses an advantage with respect to alloy cost over the highnickel, low-chromium heat-resisting alloys.

We have discovered certain basic reasons for the embrittlement tendencies of the high-chromium, low-nickel content heat-resisting alloys, and have evolved a means for minimizing this tendency which solves the problem confronting the art.

We have observed that sand cast alloys of compositions within the range of about 20% to 30% chromium, 8% to 15% nickel, 0.05% to 0.65%

carbon, balance substantially iron, can contain but one or as many as three diflerent structural constituents, excluding carbides, depending on the balance of the chromium, nickel and carbon contents, i. e., their relative contents.

The structural or micro-constituents, other than carbide, which we have observed are: austenite, ferrite and the so-called sigma phase. Austenite will be understood by those skilled in the art to mean an iron-base solid solution which has a face-centered cubic crystal structure; and ferrite will be understood to mean an iron-base solid solution which has a body-centered cubic crystal structure. Ferrite is also referred to as alpha iron or delta iron. While the identity of the sigma phase is not well established, it is thought to be a compound of iron and chromium of the composition FeCr. However, most investigators have indicated that the sigma phase has a slight range in composition on either side of that represented by the compound. Due to the low solubility of carbon in these high' alloy steels (which is of the order of about 0.05% in the austenitic phase at temperatures of 1600 F. and below) carbides are also present in alloys containing only a relatively small amount of carbon. Each of these structural constituents has distinctive physical and mechanical properties, and the mechanical characteristics of the alloys vary, depending on the particular constituents present.

We have found that one important essential in the production of alloys of high ductility at temperatures fromabout 1200 F. to 2000 F. and of high retained ductility at room temperatures after exposure for an extended period within the above-stated temperature range is that the alloy composition have an austenitic matrix structure or a face-centered cubic crystal lattice. However, we have also discovered that it is of utmost importance to have both barbon and nitrogen present in the alloy within certain ranges in order to provide the desired degree of ductility and strength properties for these austenitic alchromium and nickel of the alloy. In other words, we have found that by properly balancing the percentages of chromium, nickel, carbon and nitrogen in our heat-enduring or heat-resisting alloy, we are able to produce an alloy for the purpose intended which is markedly superior to prior art alloys for the same purpos'es. Moreover, by properly proportioning the said elements with relation to each other, we are able to produce an alloy which is much less subject to embrittlement than prior art alloys and which possesses superior ductility or strength or both superior'ductility and strength. Likewise, we can produce such an alloy which will have superior ductility both at elevated temperatures and at room temperatures after being subjected to heat. In addition to this, alloys made in accordance with our invention possess superior creep resistance to most prior art alloys of the type indicated.

An important feature of this invention is that we have provided austenitic high-chromium, lownickel content heat-enduring alloy steels which have a carbon range of 25% to .44%, nitrogen 0.02% to 0.15%, chromium 21% to 29%, nickel from 11% to 14%, with the chromium, carbon and nickel in such proportions that the chromium minus sixteen times the carbon does not exceed about 1.7 times the nickel content to give a balanced alloy and to insure an austenitic structure and further that the carbon and nitrogen are held in definite relation so that the carbon plus one-half the nitrogen is within the range of 26% to .45% in order to regulate the ductility and strength of the alloy.

Another important feature of the present invention is that we have provided austenitic highchromium, low-nickel content alloy steels of a preferred narrower composition range, containing carbon in the range of 25% to 35%, nitrogen 0.02% to 0.15%, chromium 23% to 27%, nickel 11.5% to 13%, with the relation among the carbon, chromium and nickel contents being such that the chromium less sixteen times the carbon does not exceed about 1.7 times the nickel and ,the carbon plus one-half the nitrogen within the ferrite, 8" brittle constituent or sigma phase and "C" carbides.

Figure .1 illustrates, in nomographic chart form, the approximate relation of the composition limits or boundaries between alloys which are outside the scope of our invention and the substantially austenitic alloys of our invention. Lines A, B and C have been drawn for illustrative purposes. For example, line A at a carbon content of 0.30 per cent and a chromium content of 24 per cent, indicates that a minimum of about 11.6 per cent of nickel must be present to insure a substantially austenitic alloy. Similarly this line indicates that if the nickel content is fixed about 11.6 per cent the maximum per cent of chromium which can be present in a substantially austenitic alloy is 24 per cent. Similar relations are shown by lines B and C as will be discussed more in detail hereinafter.

Figure 2 represents a photomlcrograph of the structure of an alloy containing ferrite and austenite, the composition of which is not within the composition limits of the present invention and wherein the constituents are indicated.

Figure 3 represents a photomicrograph of the structure of the same alloy as that of Figure 2 after havin been heated at 1600 F. The former ferritic areas are transformed into a brittle constituent resulting in serious loss of ductility of the alloy.

Figure 4 represents a photomicrograph of the structure of an alloy wherein the chromium minus 16 times the carbon is greater than 1.7 times the nickel, so that the alloy is not within the scope of our invention. It lacks ductility, due to the formation of a brittle constituent FeCr, or sigma phase.

Figure 5 represents a photomicrograph of the 'as-cast" structure of an alloy having a wholly austenitic matrix, which alloy is of a balanced composition in accordance with our invention and has carbon and nitrogen contents falling within our speciallimits.

Figure 6 represents a photomicrograph of the structure of the same alloy as shown in Figure 5 after the alloy has been heated for 48 hours at a temperature of 1600 F.

Figure 7 represents a photomicrograph of th structure of an alloy whose carbon content is above that of the special limits of our present invention, as a result of which stringers of carbides outline the grain boundaries and surround the grains.

Figure 8 shows, in nomographic chart form, the ductility and yield strength of "a8ed austenitic iron-chromium-nickel alloys made in accordance with our invention, the figure showing that the said ductility and yield strength values are functions of the carbon and nitrogen contents, as will be discussed and explained more in detail hereinafter.

Where the term aged" is used in this specification, reference is had to alloys which have been heated for 48 hours at 1600 F. and cooled in the furnace.

We have found that the composition ranges of austenite and ferrite as indicated by the investigations of the essentially carbon-free iron-chromium-nickel alloys are not applicable to the sand cast heat-resisting alloys which do contain appreciable carbon and which, through relatively slow solidification in sand molds, have a rather highly cored" dendritic structure. From our investigations, we have surmised that the carbon contained in the alloy is most largely confined to the metal last to solidify (i. e., the interdendritic material) and that the last metal to solidify is somewhat richer in chromium and probably less rich in nickel than that metal contained in the body of the dendrites, which are formed first during the process of solidification. This lack ,of homogeneity of composition from point to point in the sand cast alloy structure is in itself sufiicient to shift the composition boundaries of the phases from those indicated by the investigations of the constitution of wrought alloys. We have observed that the-carbon contained in the alloys plays an all-important role in shifting the phase boundaries of the iron-v chromlum-nickel alloys. Carbon in solid solution is indicated to increase the composition range of austenite at high temperatures. However, due to its low solubility (about 0.05% carbon) at temperatures of about 1600 F. and below in the high-chromium, low-nickel austenite, it becomes relatively inefiective at low temperatures. The carbon in excess of the amount retained in solid solution combines with chromium to form chromium carbides reported to be of the composition CHC (ratio of Cr to C being 208:12 or 16:1). In other words, each part by weight of carbon not retained in solid solution combines with and withdraws 16 parts by weight of chromium from solid solution. Thus, it is possible to maintain the chromium content constant while lowering the nickel content and still obtain an austenitic matrix by increasing the carbon content; or it is possible to maintain the nickel content constant and increase the chromium content and still obtain an austenitic matrix by increasing the carbon content, these relations being subject to certain limitations, however.

Through study of the microstructures and the ferromagnetic properties of a large number of sand cast alloys of compositions within the range of 18% to 28% chromium, 9% to 15% nickel, 0.05% to 0.65% carbon, about 1% manganese, about 1% silicon and balance substantially iron, we have established the composition limits for the essentially austenitic alloys within the abovestated range. By the term essentially austeniticalloy, we mean those compositions which have a wholly austenitic matrix structure at room temperature, both as cast and after exposure at temperatures within the range of 1200 F. to 2000 F. While the preferred structure is entirely free from ferritic areas, small amounts of this constituent up to about 5% by volume may be present without rendering this type of alloy too brittle for satisfactory service and it is within the scope of this invention to utilize alloys which show from traces to about 5% ferritic areas when examined microscopically. Such ferritic areas must not be in stringers which tend to surround the austenitic areas and to form a continuous network. Ferritic areas tend to lower creep strength and to promote embrittlement at elevated temperatures and at room temperature after exposure to elevated temperatures. Therefore, the amount and distribution of such ferritic areas should be quite limited.

The composition limits which we have established for the essentially austenitic alloys containing from 0.25% to 0.45% carbon are shown by the nomographic chart, Figure 1. As previously pointed out, this chart serves to indicate the relation which must be maintained between the carbon content, the chromium content and the nickel content in order to produce a substantially austenitic matrix which, as will be shown later, is an essential requirement for the producatively high creep strength and resistance to the tion of an alloy of adequate ductility having relside the limits of the essentially austenitic alloys, and also those of alloys within the austenitic limits but of different carbon contents, is

given by the micrographs of Figures 2 to 7. The chemical compositions of the alloys whose structures are shown in Figures 2 to 7, are given in Table I.

Figure 2 shows the microstructure of alloy AJ (see Table I for composition) after heating to 2050 F. for 30 minutes and quenching in water. Thematrix is austenite while the fairly continuous stringer-like phase is ferrite. In a shorttime tension test, the alloy in the above-stated condition showed an elongation in 2 inches of 38.5% and reduction of area of 57.2%.

Figure 3 shows the microstructure of alloy AJ after an identical heating cycle to that of Figure 2 (2050 F. for 30 minutes, water quenched) plus an additional heating to 1600 F. for '48 hours, followed by cooling in air. The microstructure consists of an austenitic matrix and a fairly continuous network of brittle constituent (FeCr, or the sigma phase). In a short-time tension test conducted at room temperature, this material showed elongation and reduction of area values of but 1% and 1.1%, respectively. The short sojourn at the intermediate temperature of 1600 F. has definitely embrittled the al- 10y. It is clearly indicated that it is the ferritic phase shown in Figure 2 which has transformed over to a brittle constituent during the short sojourn at 1600 F.

TABLE I.C'omposition of alloys for which photomicrogfraphs are shown in Figures 2 to 7 Chemical analysis Fig. No. Heat No,

C N Cr Ni Si Mn AJ 0. 07 0. 073 25. 32 12. 99 l. 24 0. 94 AM 0. 31 0. 062 26. 33 9. 70 l. 26 0. 94 A81 0. 33 0. 032 24. 18 12. 36 0. 83 0. 96 A G 0. 135 0. 078 25. 53 12. 50 l. 07 0. 78

Figure 4 shows the microstructure of alloy AM after being heated for 48 hours at 1600 F. This alloy contained 0.31% carbon in combination with about 26% chromium and about 10% nickel. Because of the fact that the chromium minus 16 times the carbon is more than 1.7 times the nick e1, this alloy is not within the composition range of our invention. The matrix is essentially austenitic and contains some small carbides which are hardly discernible at 500 magnification. Located between the dendritic branches of the cast structure are discontinuous eutectic carbides and stringer-like islands of brittle constituent. In the as cast condition, the areas of brittle constituent in Figure 4 were largely ferritic, and in a short-time tension test at room temperature, the alloy showed elongation and reduction of area values of 10.5% and 11.3%, respectively.

However, material in the condition of that of. Figure 4 showed elongation and reduction of area values of but 3.0% and 2.9%, respectively. Hence, it is again shown that embrittlement is induced by the transformation of high chromium ferrite into a brittle constituent at intermediate temperatures.

Figure 5 shows the microstructure of alloy AS! in the as cast" condition, said alloy being within the scope of our invention. The chemical composition of this alloy is noted from Table I to be not far difierent from that of alloy AM, which has been discussed in connection with Figure 4. The carbon contents of AS! and AM are about equal while the chromium content of AS! is about 2% lower andthe nickel content about 2%% higher than that of alloy AM. The microstructure of alloy ASI shows the matrix to be substantially austenitic and to contain rather uniformly dispersed areas of eutectic carbide.

Figure 6 shows the microstructure of alloy ASI after heating at 1600 F. for 48 hours. The structure of Figure 6 is seen to diifer from that of Figure in that heavy precipitation of small carbide particles is shown outlining the dendritic branches of the cast structure of Figure 6. While the precipitation of carbide from the super-saturated austenite results in some drop in ductility, it is not nearly so damaging as the drop caused by the formation of the brittle constituent from ferrite at 1600 F. as was shown for alloy AM. In shorttime tension tests at room temperature, alloy AS! in the as cast condition showed elongation and reduction of area values of 21.0% and 26.3%, respectively, while after heating at 1600 F. for 48 hours, the elongation and reduction of area values were 13.0% and 18.6%, respectively.

Figure 7 shows the microstructure of alloy AG after heatingat'1600" F. for 48 hours, said alloy being outside of the composition range of our invention due to its high carbon content. The matrix of the alloy is austenitic, but rather continuous stringer-like carbides are seen to outline the dendritic branches of the cast structure. Some small carbides, precipitated from the austenite on heating, are observed within the matrix. In short-time room temperature tension tests, alloy AG in the as cast condition showed elongation and reduction of area values of 5.5% and 4.8%, respectively, while after being subjected to a temperature of 1600 F. for 48 hours, the alloy showed these same respective properties to be but 1.0% and 0.8%.

Thus, while the matrix of alloy AG is wholly austenitic, it is too high in carbon and the structure contains too much of carbides segregated to the grain boundary for an alloy of satisfactory ductility and for these reasons this alloy is outside the scope of the present invention.

From the above-cited examples, it is apparent that rather small compositional variations in high-chromium, low-nickel, carbon-containing alloys can promote phase changes which have a potent eifect on the ductility of the alloys; compositions outside the region of essentially austenitic alloys are low in ductility after exposure at intermediate temperatures of about 1200 F. to 1650 F., due to a transformation of high-chromium ferrite to a brittle constituent, FeCr, or sigma phase. Control of the carbon content of the alloys which are within the austenitic region has been found to be essential for the promotion of high ductility.

From the study of the microstructures of numerous alloys'containing carbon in the range of 25% to .45%, chromium in the range of 21% to 29% and nickel in the range of 11% to 14%, it has been found that the relation of the relative concentrations of these elements to form a wholly austenitic matrix can be expressed by the mathematical equation (Jr-16C wherein F is a ratio factor which should not exceed about 1.7 if the alloys are to have a wholly austenitic matrix. The above equation is graphically illustrated in Figure 1. The expression Cr16C indicates the chromium available for solution in the austenite (or ferrite) and it has been found that it is the ratio of this chromium to the nickel which largely determines the structure of the alloy. The value 160' is used to represent the amount of chromium in combination as carbides in which the weight of chromium is approximately sixteen times that of the carbon. Elements which are functional equivalents of chromium may replace part of the chromium in the equation above or may supplement the chromium, and such elements are generally classed as ferrite promoters and include silicon, aluminum, vanadium, molybdenum, tungsten, columbium, titanium.

Elements which promote the austenitic structure, as does nickel, may replace in part or supplement the nickel in the equation and such elements include manganese, copper and cobalt. Nitrogen is also an austenite promoter and serves to supplement nickel and related elements in rendering the alloy structures austenitic.

The application of the equation above to the alloys given in Table I shows that alloy AM gives a ratio factor greater than 2. This is in agreement with the discussion which points out that this alloy does not have a satisfactorily balanced composition. On the other hand, the application of this equation to alloy A81 in Table! gives a ratio factor of 1.53 which is well below the maximum ratio of 1.7 and, as the previous discussion shows, this alloy has the desirable structure and will later be shown to have the desirable properties.

The highest-ductility, austenitic alloys producible are essentially carbon free. Such alloys would have but little application in high temperature engineering since carbon is essential for promoting strength properties; also, essentially carbon-free, low-nickel content alloys would necessarily be of relatively low chromium content or high nickel content in order to have an essentially austenitic structure. Furthermore, essentially carbon-free alloys of the type under consideration are diflicult to prepare as castings.

It has been found by practical experience and experimental measurements that a minimum chromium content of about 22% chromium is essential for a high degree of resistance to oxidation and sulphidation corrosion at elevated temperatures, while nickel adds some measure of oxidation resistance it increases the susceptibility of the alloys to sulfur attack. It is, therefore, desirable to have a certain ratio of chromium to nickel in order to promote a high degree of resistance to corrosive fuel gas atmospheres. Fortunately, carbon combines with chromium to form carbides. hence withholding a certain amount of carbon from the solid solution and thereby allowing a higher total chromium content with the retention of an austenitic structure than if little or no carbon were present in the alloy. The presence of a substantial carbon content is, therefore. highly desirable, and the preferred lower limit of carbon content for the heatresisting alloy compositions of our invention is about 0.25% carbon.

We have discovered that nitrogen acts similarly to carbon in promoting increased strength properties, especially under conditions of rapidly applied loading, as in short-time tension tests. However, both carbon and nitrogen tend to decrease ductility; hence, there is an upper limit to the amount 'of carbon and nitrogen which may be used in alloys when ductility is an important requirement. It has been found that in alloys of this class too high nitrogen contents are not desirable. Therefore, our preferred upper limit on nitrogen content is about 0.15%, since high percentages of nitrogen are not retained in solid solution but are given off on solidification and, as stated, results in porosity or sponginess. It is impractical to make these alloys entirely free from nitrogen and, as a result, alloys of this class will have a nitrogen content of at least .02%. However, the nitrogen content alone is not so important in our alloys as is the nitrogen content in relation to the carbon content. More speciflcally, the sum of the carbon plus one-half of the nitrogen is an important factor in determining ductility of the alloys after aging and the sum of the carbon plus two times the nitrogen is an important factor in determining the yield strength of the alloys after aging, both of which facts are shown in the nomographic chart of Figure 8. Thus, there are advantages in having substantial amounts of nitrogen in our alloys because nitrogen is an effective strengthening element and, as compared with carbon, nitrogen suflicient to promote a given increased strength has a less harmful effect in lowering the duetility.

We have observed that the room temperature tensile properties of the austenitic alloys after being subjected to elevated temperatures for extended periods of time are quite well reflected by the room temperature tensile properties of material which has been exposed for 48 hoursat 1600 F., to eifect aging. We have, in certain instances, used this latter short exposure treatment, followed by room temperature tension tests, to obtain a rapid evaluation of the stability and of the strength and ductility properties.

From the results of room temperature tension tests on a great number of as cast austenitic alloys after an exposure of 48 hours at 1600 F., we have been able to develop the relationship which exists between the carbon and nitrogen contents of the alloys and the strength and ductility properties. Certain of these data relative to the yield strength (yield strength values correspond to the unit load at .2% deformation determined in accordance with the A. S. T. M. recommended oifset method) and elongation are presented by the nomographic chart of Figure 8. The nomographic chart of Figure 8, which illustrates a feature of our invention, shows the values of yield strength and elongation which will be obtained with austenitlc iron-chromium-m'ckelcarbon-nitrogen alloys with any combination of carbon and nitrogen between the ranges of 0.25% to 0.45% carbon and 0.03% to 0.17% nitrogen when the specimens are heated for 48 hours at 1600 F. and then tested in tension at room temperature.

The test results were obtained on materials cast to the form of the Battelle-Alloy Casting Research Institute test block which has been described in an article The Alloy Casting Research Institute Test Block for Heat-Resisting Alloys by O. E. Harder, published as preprint 29 and presented at the June, 1939, meeting of the American Society-for Testing Materials.

While most of the data and discussion presented relate to the simple iron-chromium-nickel alloys containing about 1% silicon and 1% manganese, other elements, such as molybdenum, tungsten, cobalt, columbium, titanium and aluminum are added at times for various reasons. We have discovered that certain of these elements may be considered as functional equivalents of chromium and others as functional equivalents of nickel in their structural formation tendencies, and that when used as alloy additionsadjustments must be made in the chromium and/or nickel contents from that indicated by Figure 1 to obtain an alloy with an essentially austenitic matrix structure. Such alloying elements as molybdenum, titanium, columbium, tungsten and vanadium are considered as having stronger carbide formation tendencies than chromium, hence if present more of the chromium would be free to go into solid solution; which would necessitate lowering the chromium content or raising the nickel content over that indicated by Figure 1 for essentially austenitic alloys. If any of these elements were added in amounts greater than that required to combine with the carbon present, they would go into solid solution and be equivalent to an increased amount of chromium in affecting the structure.

Silicon and aluminum when present are in solid solution and promote ferrite formation; they can be compensated for by lowering the chromium content or increasing the nickel con- I elements.

It is within the scope of this invention to substitute within limited ranges for the chromium and nickel contents, any elements which are functionally equivalent to chromium or nickel and the above-indicated composition ranges may be modified somewhat by such substitution of equivalent elements.

While certain features of our invention are disclosed in the nomographic charts, Figures 1 and 8, it is within the scope of our invention to make use of special alloying elements with strong carbide and/or nitride formation tendencies in order to alter such mlcrostructural characteristics as form, distribution and stability of carbides and/or nitrides so as to further enhance the ductility and stability of ductility of the alloys.

It is sometimes advantageous in melting scrap stock of our heat-resisting alloys which would give a casting of too high a carbon or nitrogen content, to render a part of the content of these elements less active so as to produce a casting of higher ductility both as cast and after heating to higher temperatures. It is, therefore, desirable in some instances to fix a portion of either the carbon or the nitrogen or of both of these elements.

' For example, if the carbon content of the casting is higher than the desirable content to give the required ductility as shown on the nomographic chart of Figure 8, it is within the scope of our invention to add an element or elements which will fix the carbon and render it inactive so that it will not form the chromium carbides which, when present in too great an amount, cause the steel to be embrittled. For this purpose, we may use elements which are stronger carbide formers than chromium selected from the group consisting of columbium, titanium, tantalum and vanadium. Satisfactory results have been obtained with columbium. We are aware that both columbium and titanium have been used in low carbon stainless steels; for example, in stainless steels containing about 18% chromium and 8% nickel to combine with the carbon and that it is the customary practice to use the columbium or titanium in certain fixed ratios so that there-is enough to combine with all the carbon; for example, it has been proposed to use from six to ten times as much columbium, tantalum, or titanium as the carbon content of the steel. We do not make use of this high ratio of the carbide fixing elements as has been proposed by others. On the contrary, we use a relatively small amount of the carbide fixing elements in proportion to the carbon content of our alloy, the purpose being to fix only a part of the carbon in a more stable form or of improved distribution so as w modify the effect of the carbon and to obtain improved ductility.

Since columbium combines in the ratio of about six parts of columbium to one part of carbon, if we use about one-half percent of columbium, this may be assumed to combine with about .08% carbon and give us an alloy having a ductility greater than would be obtained without the use of the columbium. For example, this amount of columbium should make it possible for us to use a steel containing about 38% carbon and get the same elongation which we would get in a steel containing only about .30% to 34% carbon as disclosed on the nomographic chart of Figure 8, while having little or no influence on the strength properties of the 0.38% carbon alloy.

It is advantageous at times to make melts of somewhat higher carbon content but to add an alloying element as just disclosed so as to modify the effect of the carbon and to obtain improved ductility, as indicated by the per cent elongation in tension tests.

The nomographic chart of Figure 8 shows that as the nitrogen content increases, for alloys of a fixed carbon content the yield strength increases but the elongation decreases. For this reason, it is sometimes desirable to render all or part of the nitrogen inactive and for this purpose we contemplate the addition of elements which form stable nitrides, and elements selected from the group consisting of zirconium, titanium, boron and aluminum may be used. Especially good results have been obtained with the use of zirconium. These elements are all ferrite formers and they tend to fix nitrogen, which is an austenitizing element, and for that reason they have to be used in certain limited proportions so as-to maintain the desirable austenitic matrix of the alloy, both in the as cast condition and after being subjected to elevated temperatures.

Short-time tension tests and creep tests have indicated that carbides, which have been rendered more stable and of improved form and distribution, as by the use of columbium, tend to strengthen the alloy somewhat and at the same time slightly enhance the ductility of the alloy. Similar advantages may be obtained by rendering nitrogen inactive, as by the use of zirconium, although apparently to a lesser degree.

Broken lines have been drawn on the nomographic chart represented by Figure 1 to illustrate the balanced compositions of our invention. Line A represents an alloy containing 5 .30% carbon, 24% chromium as a maximum, and about 11.6% nickel as a minimum, which are the values for obtaining a structure which is essentially austenitic in this type of alloy. In practice, however, it is advisable to use somel what higher nickel than this chart would show in-order to allow for some segregation in the alloy and to give assurance that it will be essentially austenitic.

Line "B" shows the. relation for an alloy con- 15 taining .30% carbon, 26% chromium and indicates that a nickel content of about 12.75% will be required to make this alloy essentially austenitic.

Line "C" is for an alloy containing 45% car- 20 bon, 26% chromium and about 11.5% nickel. In a similar way, the various alloy compositions can be selected from this chart.

So far as we are aware, we are the first to point out the importance of obtaining a balanced composition in order to insure a substantially austenitic structure and a structure having the qualities requisite for high temperature service.

In a similar way, the nomographic chart of 30 Figure 8 illustrates the relations between the carbon and nitrogen content of alloys of the type in question and the yield strength or elongation values of such alloys. These relations have been found by extensive research. More 35 specifically, this research has shown that certain definite carbon and nitrogen contents must be maintained in an iron-chromium-nickel alloy in order to obtain the desired yield strength or elongation values of the alloys after aging.

40 Line A shows that this strength value can be obtained by nitrogen content of .11% and a carbon content of about .28% and result in an alloy having an elongation of about 12%.

Line "3 shows that this same yield strength 45 can be obtained by using a nitrogen content of .05% and a carbon content of .41%, but that the elongation is only between 5% and 6%.

From the examination of a large number of alloys within the range of composition for car- 50 bon, chromium, nickel and nitrogen previously disclosed, it has been found that the yield strength of the alloys after aging at 1600" F. for 48 hours, can be expressed by the formula YS=10.5+62.5(C+2N) wherein YS represents 55 the yield strength as expressed in 1000 p. s. i. units. The equation above can be transformed to give the following:

It has further been found that the ductility as measured by the elongation in a 2 inch gage length can be related to the composition of the alloys when the composition is maintained in the previously disclosed composition range by the following equation:

which equation can be transformed to the following:

As indicated, the above equations have been arrived at by extensive research. However, we desire it understood that these equations are more or less approximate and are subject to some variation. The equations are merely in- 5 troduced for illustrative purposes and while they are generally correct we do not desire to be rigidly bound by them. This is also true regarding the nomographic chart of Figure 8 which serves to present graphically the solution to these 10 equations for the range of carbon and nitrogen contents within the scope of our invention.

Illustrations of the relation of the ductility as a function of the carbon and nitrogen contents are shown by lines E and "D" in the nomographic chart of Figure 8. Line E is drawn through the point representing 8% elongation, 15% nitrogen and .32% carbon. This represents a high nitrogen analysis. Line D" also goes through the point representing 8% elon- 0 gation and through .04% nitrogen and .376 carbon. This represents analysis in which the nitrogen is low but the carbon has been correspondingly increased. The said equations are for the purpose of showing the relation of carban and nitrogen contents to the ductility of alloys within the above disclosed composition range when the specimens are tested at room temperature after aging 48 hours at 1600 F. or to represent the ductility of the alloys after service at elevated temperatures.

Creep tests have been made on numerous alloys of the general class of high-chromium, lownickel of which some were within our specified composition range, some were near our range and others were definitely outside of our specific range and ratios for one or more reasons. Some of the alloys which have been tested in creep had a wholly austenitic matrix while others had a relatively large amount of ferrite. These tests have been made at different temperatures, such as 1400 F., 1600 F. and 1800 F. and for different lengths of time up to over 1500 hours and at different loads at all of these :ifferent temperatures. Experience indicates that tests 15 at 1800 F. are the most informative for this type of alloy because this temperature is on the high side of the usual operating temperatures and tests at this temperature seem to be more discriminative among the diiferent alloy compositions, probably because reactions, transformations or other changes take place more rapidly and go more nearly to equilibrium conditions at the higher temperature.

Testing engineers and metallurgists recognize three stages in the time-elongation curve of specimens subjected to load at elevated temperatures, as in creep testing when the load and temperature are kept constant. The firs-t stage is made up of elastic plus plastic flow. The sec- 0nd stage, in which the rate of elongation or rate of creep remains essentially constant, is generally the basis for design calculation and for extrapolation to indicate the service life or to calculate creep rates. In the third stage, the rate of elongation is increasing and will continue to increase until fracture occurs causing failure of the element. From the creep rate during the second stage, it is customary to extrapolate and to express the rate for given temperatures and loads in percent, such as 1% in 10,000 hours or .1% in 1000 hours. The time required to reach and pass through the different stages of creep is of importance and is influenced by the material, temperature and load or unit stress.

For illustrative purposes only, the results or TABLE 1I.Illustrative chemical compositions and properties of alloys of our invention Chcmiml composition Elong.-%!2" Ratio .Alloy No. [actor c N Cr Ni Si Mn (a) 2 IExper.

AS-l-D 0.131301132243124 0.83090 1 52 11.4 13 AS+'D-. 0.310.15 25.3120 0.910.99 l 69 8.8 8 0 .-\Ql)(c). l0.38 0.073 23.9 11.8 0801.05 1 51 6. 7 8 5 C 1 C (u) Ratio faclor= (0) Calculated from E=34.46fi.6( (c) This alloy also contained 0.97% Cb.

Yield strength (rccp rates (1000 p. s. i.)

Alloy No. C t

Calculated Expert aig? Stress 52% (a mental pemtme p. s. 1 (b) F. %/hr.

AS-l-D 35. 1 33. 7 1800 2000 00003 AS 4D 48. 6 47. 9 1800 2000 00002 AQ-D 43.4 45.9 1800 2000 .00001 (:1) Calculated from YS=10.5+62.5 (C+2N).

(b) Extrapolation of creep rates at 500 hours to rates for 10,000 31132 13 gives the following: ASl-D =0.3%, AS-4D =0.2% and AQ-D Examination of the data in Table II shows that all of the chemical compositions are within our specified ranges for carbon, chromium, nickel and nitrogen. Microscopic examination of the specimens showed that these alloys were characterized by a wholly austenitic matrix and, as shown in the table, the ratio factors are all less than 1.7, as we have specified for alloys having a wholly austenitic matrix.

'The agreement between the calculated and the experimentally determined values for the per cent elongation (E) in tension tests on specimens aged 48 hours at 1600 F. is as good as is to be expected for this type of relation and the same may be said regarding the yield strength values. Alloys AQ-D illustrate a special feature of our findings in that a strong carbide former, such as columbium in this alloy, can serve to render the carbon less potent for reducing the ductility and it will be noted that alloy AQ-D showed some 28 per cent higher elongation in the test than the calculated value. As previously po nted out, carbides which have been rendered less active as by the use of strong carbide formers, such as columbium, have improved strength properties and it will be noted that alloy AQ-D containing 0.97% columbium had a higher yield strength (0.2% deformation) than the calculated value and that this alloy showed unusually good creep resistance, as shown by a creep rate of only .00001%/hr. or 0.1% in 10,000 hours at a stress of 2000 p. s. i. at 1800 F.

Our researches have established that it is highly advantageous in alloys for service at elevated temperatures and under load to use materials which have a wholly austenitlc matrix and which have stable structures. We have found that the structure which is designated as ferrite, alpha iron or delta iron is disadvantageous, particularly when it is present in significant amounts and that it is especially harmful when present in stringers which surround or outline the austenite. The harmful effects of ferrite seem to result from its tendency to form the brittle sigma phase which reduces ductility and to the fact that ferritic materials have decidedly lower strengths and lower resistance to creep at elevated temperatures than do austenitic materials even though the chemical compositions would not at first sight seem markedly difierent to those who have not studied this subject. These relations help to show the impor tance of our discovery, that alloy compositions can be balanced with reference to those elements which promote the austenitic structure and those which promote the ferritic structure so as to produce alloys of the high-chromium, lownickel type with a wholly austenitic matrix, with improved ductility after exposure to elevated temperatures and with improved strength and high resistance to creep.

Structural parts made according to our invention will not fracture so readily when subjected to momentary overloads, mechanical abuse, thermal gradients or thermal shock, as the same structural parts of the prior art. It is also significant that the cost of our alloy is decidedly less than that of the high-nickel, lowchromium types which have heretofore been considered necessary in certain industrial applications.

While our invention is particularly applicable to castings, it is not limited thereto and within the composition ranges disclosed and claimed also applies to wrought or otherwise shaped articles of manufacture.

Having disclosed the nature of our invention and given illustrations of its application to the production of heat-resisting alloys, we claim the following:

1. A heat-enduring alloy containing chromium in the range of 21% to 29%, nickel 11% to 14%, nitrogen 0.02% to 0.15%, with the carbon plus one-half the nitrogen from 0.26% to 0.45% and the balance being substantially iron except for the usual contaminants in common amounts, said alloy being substantially austenitic.

2. A heat-enduring alloy containing carbon in the range of .25% to .44%, chromium 21% to 29%, nickel 11% to 14%, nitrogen 0.02% to 0.15%, the balance being substantially iron except for the usual contaminants in common amounts, said alloy being substantially austenitic with the relation among the carbon, chromium and nickel contents such that the chromium content minus sixteen times the carbon does not exceed about 1.7 times the nickel, and the carbon plus one-half the nitrogen is within the range of 0.26% to 0.45%.

3. A heat-enduring alloy containing chromium in the range of 21% to 29%, nickel 11% to 14%, nitrogen 0.02% to 0.15% with the carbon plus one-half the nitrogen from 0.26% to 0.38% and the balance being substantially iron except for the usual contaminants in common amounts, said alloy being substantially austenitic.

4. A heat-enduring alloy containing carbon in the range of 0.25% to 0.35%, chromium 21% to 29%, nickel 11% to 14%, nitrogen 0.02% to 0.15%, the balance being substantially iron except for the usual contaminants in common amounts, said alloy being substantially austenitic with the relation among the carbon, chromium content minus sixteen times the carbon does not exceed about 1.7 times the nickel, and the carbon'plus one-half the nitrogen is within the range of 0.26% to 0.38%.

5. A heat-enduring alloy containing chromium in the range oi! 23% to 27%, nickel 11.5% to 13%, nitrogen 0.02% to 0.15% and with the carbon plus one-half the nitrogen.from 0.26% to 0.38% and the balance being substantially iron except for the usual contaminants in common amounts, said alloy being substantially austenitlo.

6. A heat-enduring alloy containing carbon in the range of 0.25% to 0.35%, chromium 23% to 27%, nickel 11.5% to 13%, nitrogen 0.02% to 0.15% and the balance being substantially iron except for the usual contaminants in common amounts, said alloy being substantially austenitic with the relation among the carbon, chromium and nickel contents such that the chromium minus sixteen times the carbon does not exceed about 1.7 times the nickel and the carbon plus half the nitrogen is within the range of 0.26% to 0.38%.

7. A heat-enduring alloy containing chromium in the range of 23% to 27%, nickel 11.5% to 13%, nitrogen 0.02%v to 0.15% and with the carbon plus one-half the nitrogen from 0.26% to 0.38% and the balance being substantially iron except for the usual contaminants in common amounts, said alloy being substantially austenitic, and being further characterized by high retained ductility after being heated to elevated temperatures so that when tested at room temperature after heating 48 hours at 1600 F. it shows an elongation of at least 8% in a 2inch gage length.

8. A heat-enduring alloy containing carbon in the range of 0.25% to 0.35%, chromium 23% to 27%, nickel 11.5% to 13%, nitrogen 0.02% to 0.15% and the balance being substantially iron except for the usual contaminants in common amounts, said alloy being substantially austenitic with the relation among the carbon, chromium and nickel contents such that the chromium minus sixteen times the carbon does not exceed about 1.7 times the nlckel, and the carbon plus one-half the nitrogen is within the range of 26% to .38%, said alloy being further characterized by high retained ductility after being heated to elevated temperatures so that when tested at room temperature after heating 48 hours at 1600 F. it shows an elongation of at least 8% in a 2-inch gage length.

9. A cast heat-enduring alloy containing carbon in the range of 0.25% to 0.35%, chromium 23% to 27%, nickel 11.5% to 13%, nitrogen 0.02% to 0.15%, the balance being substantially iron except for the usual contaminants in common amounts, said alloy being substantially austenitic and the carbon, chromium and nickel being in the proportions that the chromium minus sixteen times the carbon does not exceed about 1.7 times the nickel, and the carbon and nitrogen being in such relation that the carbon plus onehalf the nitrogen is in the range of 0.26% to 0.38%, said alloy being further characterized by high ductility as cast and after being subjected to heat, by high yield strength and by high resistance to flow under load at elevated temperatures.

10. A cast heat-enduring alloy having substantially the composition of carbon 0.33%, nitrogen 0.03%, chromium 24.2%, nickel 12.4%, silicon 0.83%, manganese 0.96%, the balance being substantially iron, except for small amounts of usual contaminants and said alloy having substantial- 1y an austenitic structure; said alloy being further characterized by high ductility both as cast and after being subjected to heat, and by high resistance to creep at elevated temperatures,"

said alloy showing an elongation of over 10% on a 2-inch gage length when tested at room temperature after aging 48 hours at 1600 F., and a creep rate of not over .00005%/hour at 1800" F. under a load of 2000 p. s. i.

11. A cast heat-enduring alloy having substantially the composition of carbon 0.31%, nitrogen 0.15%, chromium 25.3%, nickel 12.0%, silicon 0.91%, manganese 0.99% and the balance being substantially iron, except for small amounts of usual contaminants, and said alloy having substantially an austenitic structure; said alloy being further characterized by high ductility both as cast and after being subjected to heat, by ,a high yield strength and by high resistance to creep at elevated temperatures, said alloy showing an elongation of about 8% on a 2-inch gage length when tested at room temperature after being aged for 48 hours at 1600 F., a yield strength of over 45,000 p. s. i. (2% deformation) in the aged condition, and a creep rate of not more than .00005%/hour at 1800 F. under a load of 2000 p. s. i.

12. A cast heat-enduring alloy having substantially the composition of carbon 0.38%. nitrogen 0.073%, chromium 23.9%, nickel 11.8%, silicon 0.86%, manganese 1.05%, columbium 0.97% and the balance being substantially iron, except for small amounts of usual contaminants, and said alloy having substantially an austenitic structure and having its ductility improved by the columbium combining with some of the carbon to render it less active and less effective in reducing the ductility; said alloy being further characterized by high ductility both as cast and after being subjected to heat, by high yield strength and by high resistance to creep,at elevated temperatures, said alloy showing' an elongation of about 8% on a 2-inch gage length when tested at room temperature after being aged for 48 hours at 1600 'F., a yield strength of about 45,000 p. s. i. in the aged condition, and a creep rate of not more than .00005%/hour at 1800 F. under a load of 2000 p. s. i.

13. An article of manufacture for use at elevated temperatures, comprising an element having substantially the composition of carbon .25% to .35%, chromium 23% to 27%, nickel 11.5% to 13%, nitrogen .02% to 15%, the balance being substantially iron, except for usual contaminants in common amounts; said element having substantially an austenitic structure and the carbon, chromium and nickel being in the proportions that the chromium minus sixteen times the carbon does not exceed about 1.7 times the nickel, and the carbon and nitrogen being in such relation that the carbon plus one-half the nitrogen is in the range of .26% to 38%, said element being further characterized by high ductility as cast and after being subjected to heat, by high yield strength and high resistance to flow under load at elevated temperatures.

14. An article of manufacture for use at elevated temperatures, comprising a cast element having substantially the composition of carbon 25% to 35%, chromium 23% to 27%, nickel 11.5% to 13%, nitrogen .02% to .15%, the balance being substantially iron, except for usual contaminants in common amounts; said element having a substantially austenitic structure and the carbon, chromium and nickel being in the proportions that the chromium minus sixteen times the carbon does not exceed about 1.7 times the nickel, and the carbon and nitrogen being in such relation that the carbon plus one-half the nitrogen is in the range of 28% to 38%, said element being further characterized by high ductility as cast and after being subjected to heat, by high yield strength and high resistance to flow under load at elevated temperatures.

OSCAR E. HARDER. JAMES T. GOW.

CERTlFICATE or CORRECTION.

Patent No. 2,225, 659. December 3, 191m.

OSCAR E. HARDER, ET AL- It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 7, first column, line 59, after "aging." insert the following paragraph Having selected one of these values, then, a number of possibilities exist with reference to the nitrogen and carbon contents; for example, for a yield strength of the alloy in the aged condition of h2,ooo p31,, the alloy-may have .07sz nitrogen, .56

, of carbon and show an elongation of 8%.

and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent 01- i'ice.

Signed and sealed this hth day of March,A. D. 19in.

Henry Van Arsdale (Seal) Acting Commissioner of Patents. 

